1. Field of the Invention
This invention relates to a method of monitoring and detecting the idealized end of a ball bonding procedure with automatic termination of the ball bonding procedure in response to such detection.
2. Brief Description of the Prior Art
In the process of semiconductor fabrication, one of the operations involves bonding of wire between pads on the chip and lead fingers of leadframes. This bonding operation generally involves the formation of a thermosonic ball bond to the pad and then threading of the wire attached to the ball bond through a capillary with subsequent formation of a stitch bond to the leadframe lead finger at the other end of the wire with subsequent severing of the stitch bond and wire. The bond of the ball is sometimes referred to as a weld. In the formation of the thermosonic ball bond portion of this operation, the wire is generally pre-heated to a temperature minimally below the flow point of the wire material, generally gold and generally to about 250 degrees C. As the ball contacts the device bond pad, it quickly heats to the device bond pad temperature. The ultrasonic frequency used is optional, it being understood that the industry standard has been 60 KHZ with an improved high frequency version operating at 116 KHz. The ball is then welded to the pad with the continued heating of the ball by ultrasonic energy radiation and by the application of a compressive force between the ball and the pad by the capillary through which the wire is threaded. It is believed that the gold ball changes state as a result of the welding operation, going from a molten state to a crystalline state.
This prior art bonding procedure assumes an ultrasonic power requirement being the same for all ball bonds. Since the temperature of the ball prior to the final bonding can vary from ball to ball (and the flow temperature can also vary due to variations in the wire composition), an underbonding or overbonding situation can be created. As opposed to a fully bonded state when neither an overbonding or underbonding condition exists. In the case of an underbonding, the connection may be inadequate with low intermetallics and cause failure of the device by reason of the bond coming apart at a later time. In the case of an overbonding, the ball size and height are increased, thereby causing a higher standard deviation on ball shear and percent of intermetallics. Also, with increased ball size, there is the possibility of short circuits and breakage of the glass passivation disposed over the bond pad occurring due to flow or other movement of the ball off of the pad and onto an adjacent pad or contact with a wire connected to an adjacent pad during formation of the thermosonic bond. It is apparent that when there is some degree of misalignment between the ball and the pad, the above described problem is exacerbated.